Conventionally, there have been widely recognized needs for efficient use of limited energy resources and energy saving for prevention of global warming. Today, energy demand is met by thermal power generation in such a manner that thermal energy is converted into electric power energy.
However, coal and oil required for the thermal power generation are resources of which reserves are finite, so that new energy resources are now necessary to replace the coal and oil with. Given this factor, attentions have been drawn to a fuel cell which chemically generates power with use of hydrogen for fuel.
The fuel cell has two electrodes and an electrolyte disposed between the electrodes. In the anode, supplied hydrogen is ionized to be a hydrogen ion which travels in the electrolyte toward a cathode. In the cathodes, supplied oxygen and the hydrogen ion which has traveled in the electrolyte, are reacted with each other to generate water. As electrons generated in accompaniment with the ionization of hydrogen are moved from the anode to the cathode through wiring, whereby an electric current flows to generate electricity.
The fuel cells are classified into four types mainly depending upon differences in the electrolyte. These four types are a Solid Oxide Fuel Cell (SOFC) which uses an ion-conductive ceramics for the electrolyte; a Polymer Electrolyte Fuel Cell (PEFC) which uses a hydrogen ion-conductive polymer membrane for the electrolyte; a Phosphoric Acid Fuel Cell (PAFC) which uses a highly concentrated phosphoric acid for the electrolyte; and a Molten Carbonate Fuel Cell (MCFC) which uses an alkali metal carbonate for the electrolyte. Of these, the Polymer Electrolyte Fuel Cell (PEFC) having a low operating temperature of 80° C. has been particularly in progress of development.
The polymer electrolyte fuel cell has a structure composed of an electrolyte layer having on a surface thereof a catalyst electrode; a separator which nips the electrolyte layer from both sides thereof and is grooved for supplying hydrogen and oxygen; and a power collector plate for collecting electricity generated on the electrode. Improvement has been made in the separator as well as the electrolyte layer.
The requirements to be fulfilled by the separator include: high electrical conductivity; high hermeticity against fuel gas and oxidizer gas; and high resistance to corrosion at the time of oxidation-reduction reaction of hydrogen as well as oxygen.
In order to meet these requirements, the following separator materials have been used.
One of the most frequently used materials is fine-grained carbon which is excellent in electrical conductivity, corrosion resistance, and mechanical strength, and is also higher in workability and lighter in weight. However, the fine-grained carbon is susceptible to oscillation and shock, and needs to be subjected to cutting process which leads to an undesirable increase in the processing cost. It is also necessary to perform additional treatment thereon to attain impermeability to gaseous substances.
Moreover, synthetic resin is also used, including thermosetting resin such as phenol resin and epoxy resin. The synthetic resin, although it is advantageous in terms of cost reduction, offers poor dimensional stability and low electrical conductivity.
From viewpoints of electrical conductivity, workability, hermeticity, and the like, metal has been more frequently used. The metal used mainly includes titanium and stainless-steel. However, the metal is susceptible to corrosion, and metal ion tends to be taken into the electrolyte membrane, which results in deterioration in ion conductivity. Accordingly, a separator surface needs to be plated with gold.
Further, rubber is used, including ethylene-propylene-diene rubber and the like. The rubber has low gas permeability and high sealability.
In a Japanese Unexamined Patent Publication JP-A 8-180883 (1996), a polymer electrolyte fuel cell is disclosed. In this polymer electrolyte fuel cell, as a separator is used a sheet metal including stainless-steel, titanium alloy and the like on which a passivation film is easily formed by air, and a press work is given to form a predetermined shape
Further, in a Japanese Unexamined Patent Publication JP-A 2003-297383, a separator for fuel cell is disclosed. This fuel cell separator is constituted by a metal base sheet which has, on at least one surface thereof, a first resin layer and a second resin layer formed of an admixture of resin and an electrically conductive filler. The first resin layer exhibits a volume resistivity of 1.0 Ω·cm or below. The second resin layer is smaller in volume resistivity than the first resin layer. In this way, the separator succeeds in providing enhanced power collecting capability, moldability, strength, and corrosion resistance.
Thus, also in the separator for fuel cell described in JP-A 2003-297383, a gas channel is formed by press work as in the case of the separator in the polymer electrolyte fuel cell described in JP-A 8-180883 (1996).
Moreover, as a case of not using the press work, in a separator of a polymer electrolyte fuel cell described in a Japanese Unexamined Patent Publication JP-A 2001-76748, a gas channel is formed by printing an electrically conductive material onto an electrically conductive base material. To be specific, as the electrically conductive base material is used a molded plate formed of carbon powder and a thermosetting resin as main components, and as the electrically conductive material is used carbon paste containing carbon powder as a main component.
A separator formed of rubber has low gas permeability, but low stiffness, and thus deteriorates under a high heat environment, so that warp and deformation volume are large. There arises a problem that the separator is not durable for a long-term use.
Furthermore, a future separator is demanded for reduction in thickness and weight thereof, and in order to realize such demands, it is necessary to reduce thickness and weight of a metal base and to miniaturize the gas channel. However, if the gas channel is attempted to be formed by the press work as the separators described in JP-A 8-180883 (1996) and JP-A 2003-297383, the warp and deformation are larger so that dimensional accuracy becomes poor. This deterioration of the dimensional accuracy causes decrease in yield.
The separator described in JP-A 2001-76748 can respond to the demand of miniaturization of the gas channel by printing carbon paste, but the base material is the thermosetting resin and therefore, there remains a problem that the base material itself is poor in the dimensional stability.
Furthermore, conventional separators including the above separators described in the publications need to be provided with a gasket between the electrolyte layers in order to prevent fluid from leaking.